Pulse-width modulator

Abstract

A pulse-width modulator comprising a square-wave generator which triggers a monostable multivibrator driven by a controllable constant current source, the multivibrator having a capacitor as a timing element. The output of the multivibrator controls a gate circuit to which the square wave is also applied, thereby controlling the width of the square wave output from the gate circuit. The modulator can be assembled from standard CMOS building blocks and several external components, thereby achieving a power dissipation below 10 mW. If the current source is controlled by a CMOS variable-gain amplifier, the modulator can serve as the control circuit of a clocked direct voltage converter having internal losses below 25 mW.

What is claimed is: 
     
       1. A pulse-width modulator for receiving from a square-wave generator (3) a periodic square-wave pulse signal (4a) having a fixed duty cycle and a fixed frequency, and for modulating such square-wave signal in accordance with an input signal (1a), said modulator comprising: a retriggerable monostable multivibrator (6); a timing circuit comprising a capacitor (7) and a current source (1) for charging said capacitor, said current source being controlled by the input signal; and an AND-gate (5); the square-wave pulse signal (4a) being applied to a trigger input (6a) of the multivibrator (6) and to an input (5a) of the AND-gate (5); said multivibrator (6) being triggered by falling edges of the square-wave signal, and producing an output signal which is applied to another input (5b) of said AND-gate (5). 
     
     
       2. A pulse-width modulator as claimed in claim 1, characterized in that said current source (1) comprises a transistor and a resistor connected to the emitter of said transistor, the collector of said transistor being connected to a terminal of the capacitor (7) and the base of the transistor being triggered by the input signal (1a). 
     
     
       3. A pulse-width modulator as claimed in claim 1, characterized in that the input signal (1a) is supplied by a differential amplifier (2) having an input for receiving a controlling variable voltage and another input for receiving a reference voltage produced by a reference voltage source (3). 
     
     
       4. A pulse-width modulator as claimed in claim 3, characterized in that the differential amplifier (2), the reference voltage source (3), the square-wave generator (4), the gate circuit (5), the monostable multivibrator (6) and the controllable current source (1) are comprised in a monolithically integrated circuit. 
     
     
       5. A pulse-width modulator as claimed in claim 3, characterized in that the monostable multivibrator (6), the square-wave generator (4), the gate circuit (5) and the differential amplifier (2) are implemented in CMOS-technology.